Work in progress includes synthesis and testing of several different types of analogs of the posterior pituitary hormones oxytocin and lysine vasopressin (LVP) as part of a continuing study of the relationship of structure to the biological activity of these important compouds. These types include: 1) Compounds having halogen substituents on the tyrosine residue in position 2 of oxytocin and LVP. 2) An oxytocin analog with an aromatic amino acid at position 4. 3) An LVP analog with decreased aromaticity at position 3. 4) Analogs of oxytocin characterized by formal insertion or deletion of methylene groups at position 6 instead of, or as well as, at position 1. 5) An oxytocin analog containing a different heterocyclic amino acid in place of proline at position 7.